A Rare Case of Anomalous Left Anterior Descending Artery with a Large Interarterial CT Angiographic Study

Case Report. A 50-year-old Caucasian male with no significant past medical history presented to our institution with sudden onset of shortness of breath. He admitted to having exertional chest pressure and shortness of breath for a few years. His risk factors include family history of coronary artery disease and a forty-pack-per-year smoking habit. Physical examination revealed an S3 gallop with bibasilar crackles, confirming the diagnosis of congestive heart failure. A chest X-ray revealed pulmonary edema with bilateral pleural effusions. On two-dimensional echocardiography examination there was dilatation of the left ventricle with global left ventricular dysfunction (ejection fraction of 20%). Cardiac catheterization was performed to evaluate the coronary arteries.
On angiography, there was significant atherosclerotic disease of the coronary vasculature. The right coronary artery (RCA) had a complete proximal occlusion. The left circumflex artery had >70% stenosis (Figures 1A and B). Although a diagonal artery was visualized, it was not clear from the angiogram whether the left anterior descending artery (LAD) was occluded or absent. On further evaluation, an anomalous vessel was seen originating from the right coronary cusp (Figure 2). This vessel not only supplied septal perforators, but also communicated via a rather large collateral with the posterior descending (PDA) branch of the RCA. We designated this vessel as the LAD based on its location and septal branches.
At this point, there were two questions: (1) Was ischemia contributing to left ventricular dysfunction? and (2) What was the proximal course of the anomalous vessel? A myocardial perfusion study and coronary computed tomography angiogram (CTA) were then performed. The myocardial perfusion study showed a large area of reduced-to-absent activity in the anterior and the anterolateral wall with partial reversibility.
The coronary CTA demonstrated that the anomalous vessel did indeed course between the aorta and the main pulmonary artery (Figure 3A). A small, short vessel in the location of the LAD was visualized on the CTA. This vessel was completely occluded in the mid segment (Figure 3B). In fact, we were able to visualize this small diseased LAD coming off the left main only on the CTA. CTA was also able to identify the other lesions seen by conventional angiography. The distal collateral communication was not clearly visualized on the CTA, as the patient was tachycardic and the temporal resolution was compromised.
The patient then underwent coronary artery bypass surgery. Saphenous vein grafts were placed to the anomalous LAD, diagonal, obtuse marginal, and RCA. A left internal mammary artery graft was not used, as compression of the anomalous LAD was probably intermittent and there was no significant anatomic disease in this vessel. During surgery, an alpha agonist phenylephrine was administered to assess the severity of mitral regurgitation.² Immediately after, there was a drop in the cardiac indices along with visible worsening of apical hypokinesis and mitral regurgitation on transesophageal echocardiography. Pulmonary hypertension is strongly associated with the degree of functional mitral regurgitation.³ In some patients, the sudden appearance of it can produce pulmonary artery dilatation.⁴ We believe that the pulmonary hypertension induced by the worsening of mitral regurgitation could have increased the pulmonary artery diameter, leading to compression of the LAD and exacerbation of the wall motion abnormality observed in this patient. This confirmed our suspicion that the interarterial course of the anomalous LAD was not benign. The patient had an uneventful recovery from the surgery.

Discussion. Anomalous coronary origins are seen in 1% of patients in angiographic series.^1,5 Anomalous origin of the left main coronary artery (LMCA) from the right ostium occurs in up to 0.017–0.03% of patients.⁶ Anomalous origin of the LAD from the RCA is even more uncommon. Different courses of the anomalous vessel have been described.⁷ The course may be either anterior (interarterial), posterior to the aorta, or along the right ventricular outflow tract. This interarterial variant of the LMCA can further take one of two courses: one where the LMCA courses over the septum, and the other where it runs an intramyocardial course prior to its bifurcation. The interarterial variant of the LMCA is the most relevant of all the variants. Angina, syncope, and sudden death have been reported in patients with this condition.⁸ In our patient, the anomalous vessel not only had an interarterial course, but it also had an intramyocardial segment. This was clearly demonstrated on the CTA (Figures 4A and B).
The pathologic mechanism of the ischemia from these anomalous arteries is not exactly clear. Some of the postulated mechanisms include direct compression of the interarterial segment between the pulmonary artery and the aorta during exercise when these vessels dilate.^9,10 Inadequate flow through a narrow, unfavorable slit-like orifice of the anomalous vessel rather than an oval orifice of a normal vessel has also been described as a possible mechanism of ischemia.¹¹ Ischemia with intramyocardial bridging segments of the anomalous vessels has been reported.¹² The intramyocardial portion is prone to an abrupt cutoff of blood supply during periods of increased myocardial contractility, a phenomenon described as “milking”.¹³
In our case, the anomalous LAD not only originated from the right coronary ostium, but also communicated with the PDA branch of the RCA. Intercoronary arterial anastomoses (collaterals) have been described both in the presence and the absence of coronary stenosis. These collaterals are usually <300 µm in diameter and arise at an angle from the primary axis of the larger coronary artery. Histological appearance is that of an arteriole rather than an artery. A large communication similar to our case between the LAD and PDA (>1.0 mm) has been reported previously in the setting of a single coronary artery¹⁴ and in the setting of normal coronary anatomy.¹⁵ The histological appearance in the case described by Donaldson and Isner¹⁴ was that of an extramural coronary artery. The authors inferred two different types of communication between coronary arteries. Collateral communications as conventionally described when one of the interconnected coronary arteries has a narrowing. Alternatively, intercoronary continuity presents congenitally, and being independent of narrowing in the coronary arteries, was described in a patient by Esente et al.¹⁵ Esente et al interpreted their angiographic finding of interarterial communication to be a congenital “open-ended” continuity between the right and circumflex coronary arteries. Although it is conceivable that a large collateral could develop secondary to the obstructed right coronary artery in our patient, it is more likely that the interarterial continuity existed from birth. We believe that our patient had this rather large intercoronary communication since birth and then developed atherosclerotic disease in the coronary arteries.

Conclusion. We describe a rare case of an anomalous LAD originating from the right coronary ostium with a large communication to a completely occluded RCA. CTA in this case helped us to better define the anatomy and guide management. The interarterial course of the anomalous LAD was readily identified on CTA. This is the second case in the literature describing a large interarterial communication between the LAD and the PDA and the first case described on angiography.

References

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